Potential function for the Huntingtin protein as a scaffold for selective autophagy

Significance The normal function of the Huntingtin (HTT) protein is emerging. Here we report that selective autophagy requires an intact HTT protein in Drosophila and mouse CNS. We describe similarities in structure and binding activity between the C-terminal domain of HTT and the yeast autophagy scaffold protein Atg11, suggesting that HTT may normally function as a scaffold for various types of selective autophagy. Mice expressing an expanded repeat form of HTT also show deficits in protein clearance. Because autophagy is critical for clearance of cellular proteins, including mutant HTT, the impairment of normal HTT function by the polyQ expansion could suppress activity of the autophagy machinery. These results may have important implications when evaluating therapeutic strategies for HD. Although dominant gain-of-function triplet repeat expansions in the Huntingtin (HTT) gene are the underlying cause of Huntington disease (HD), understanding the normal functions of nonmutant HTT protein has remained a challenge. We report here findings that suggest that HTT plays a significant role in selective autophagy. Loss of HTT function in Drosophila disrupts starvation-induced autophagy in larvae and conditional knockout of HTT in the mouse CNS causes characteristic cellular hallmarks of disrupted autophagy, including an accumulation of striatal p62/SQSTM1 over time. We observe that specific domains of HTT have structural similarities to yeast Atg proteins that function in selective autophagy, and in particular that the C-terminal domain of HTT shares structural similarity to yeast Atg11, an autophagic scaffold protein. To explore possible functional similarity between HTT and Atg11, we investigated whether the C-terminal domain of HTT interacts with mammalian counterparts of yeast Atg11-interacting proteins. Strikingly, this domain of HTT coimmunoprecipitates with several key Atg11 interactors, including the Atg1/Unc-51–like autophagy activating kinase 1 kinase complex, autophagic receptor proteins, and mammalian Atg8 homologs. Mutation of a phylogenetically conserved WXXL domain in a C-terminal HTT fragment reduces coprecipitation with mammalian Atg8 homolog GABARAPL1, suggesting a direct interaction. Collectively, these data support a possible central role for HTT as an Atg11-like scaffold protein. These findings have relevance to both mechanisms of disease pathogenesis and to therapeutic intervention strategies that reduce levels of both mutant and normal HTT.